Washing machine with direct drive motor and method for its manufacture

ABSTRACT

A washing machine with a direct drive motor system includes a plastic tub, a drum rotatably mounted inside the tub and a shaft connected to the drum for transmitting a driving torque of a direct drive motor. The motor has a rotor assembly fastened to the shaft and a stator assembly fastened to the tub. The plastic tub includes a central protruding portion where a central opening of the stator assembly is centered before it is fastened to the tub, the central opening of the stator assembly presenting a plurality of centering surfaces configured to cooperate with an outside cylindrical surface of the central protruding portion of the tub

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of European Application Serial No. 15158899.3, filed Mar. 12, 2015, the entirety of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a washing machine with a direct drive motor system comprising a tub, a drum rotatably mounted inside the tub, and a shaft connected to the drum for transmitting a driving torque of said direct drive motor system, this latter comprising a rotor assembly fastened to the shaft and a stator fastened to the tub. Such known solution has the problem of a high precision alignment between the stator and the rotor, such alignment being required for guaranteeing proper air gap between rotor magnets and stator coils. Since the rotor and the stator are fastened to different components and to different areas of the rear wall of the tub, proper centering is of paramount importance for a correct working of the direct drive motor system.

Existing solutions for solving the above centering problem comprise a stator centering (before final fastening) on plastic pins provided “ad hoc” on the tub, a stator centering on a cylindrical reworked metal hub surface, or a stator centering on metal pins provided on the metal hub. All the above solutions require special features for guaranteeing a proper centering, which increase the overall cost of the appliance and the complexity of the assembly thereof.

EP 2626459 discloses a washing machine having a direct drive motor where the stator comprises fixing pins inserted into the rear wall of the tub. The use of pins and holes make the overall construction of the washing machine more complex.

EP 1580311 discloses a direct drive motor where the stator is centered by means of a cylindrical or conical protuberance which enters the bearing hole which is precisely machined in order to seat ball bearings. This known solution requires to reduce the axial distance of the ball bearings, with a direct impact on the rigidity of the drum shaft support, or to increase the axial dimension of the central portion of the bearing spider. Moreover the solution disclosed by EP 1580311 is not for washing machine having a plastic tub, since it is clearly fit for washing machines having a metal tub.

DE 10060940 discloses a washing machine which, similarly to EP 1580311, uses a metal bearing spider which cooperates with an axial protrusion of the stator during reciprocal engagement. Metal surface of the bearing spider has to be precisely machined in order to properly center the stator.

BRIEF SUMMARY OF THE INVENTION

In one aspect of the invention, a washing machine with a direct drive motor system includes a plastic tub, a drum rotatably mounted inside the tub and a shaft connected to the drum for transmitting a driving torque of a direct drive motor. The motor includes a rotor assembly fastened to the shaft and a stator assembly fastened to the tub. The plastic tub has a central protruding portion where a central opening of the stator assembly is centered before its final fastening to the tub. The central opening of the stator assembly presents a plurality of centering surfaces configured to cooperate with an outside cylindrical surface of the central protruding portion of the tub.

In another aspect of the invention, a method for producing a washing machine with a direct drive motor system is provided where the washing machine includes a plastic tub, a drum rotatably mounted inside the tub and a shaft connected to the drum for transmitting a driving force of a direct drive motor, and the motor has a rotor assembly fastened to the shaft and a stator assembly fastened to the tub. The method includes co-injecting the plastic tub on a metal reinforcing element which acts also as a central hub so that a central protruding plastic portion is created as a plastic layer on one end of the hub. The stator assembly axially slides on the plastic layer in order to center the stator assembly. A plurality of centering surfaces provided on the central opening of the stator assembly cooperates with an outside cylindrical surface of the central protruding portion of the tub during the axially sliding. The centering of the stator on the tub is made by using only specific centering surfaces of the stator provided on the edge of the central opening used for its assembly to the protruding portion of the tub.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross longitudinal section of the rear wall of a tub of a washing machine according to the invention, in which the direct drive motor system is installed.

FIG. 2 is an enlarged detail of FIG. 1.

FIG. 3 is a detail of a front view of the rear wall of the tub, where the rotor has been removed.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

With reference to the drawings, a direct drive washing machine comprises a plastic tub 10 whose rear wall 10 a presents a reinforcing star-shaped metal element 12 which forms also a central hub 12 a for mounting ball bearings 14. Such bearings 14 are used supporting a shaft 16 supporting a drum 18.

According to the invention, the rear wall 10 a of the plastic tub 10 presents an axial rear protrusion 20 having a first portion 20 a slightly tapered and a second portion 20 b which is cylindrical and whose diameter corresponds exactly to the diameter of a circumference defined by three centering curved surfaces 22 formed on an edge 22 a of a central hole of a stator 24.

On one end 16 a of the shaft 16 opposite the drum 18 a cup-shaped rotor 26 is mounted.

In order to mount the stator 24 in a correct angular position on the tub 10, the rear wall 10 a of the tub presents, on its axial protrusion 20 a radial extension 28 (FIG. 3) which corresponds to a radial seat 30 of the stator 24.

When the direct drive motor is assembled to the tub 10, first step is to insert the stator 24 on the axial protrusion 20 of the tub, so that centering surfaces 22 slide first on the tapered portion 20 a for a pre-centering operation and then it slides on a cylindrical smooth surface 21 of a portion 20 b for a final centering operation, such movement ending when the stator 24 abuts the rear wall 10 a of the tub 10. Then the stator 24, perfectly centered on the tub 10, is fixed in a known way by means of screws 32.

Next and final step is to install the rotor 26 on the end 16 a of the shaft 16.

In order to allow a precise dimension of the axial protrusion 20 of the tub 10, particularly of the plastic portion A which covers the metal hub 12 a, it is important that, whatever polymeric material is used in the injection molding process, the injection point or points is placed very close to A in order to get rid of any shrinkage effect or any surface defects.

By injecting plastic material of the tub in a central zone thereof, particularly in a central zone of the rear wall of the tub, such central zone can maintain very strict dimensional tolerances which allows a precise centering of the stator, without any need of auxiliary centering pins or reworking of coupling metal surfaces. Such solution is therefore very simple and allow to obtain a high precision in the centering process while, at the same time, maintain the use of a plastic tub with related advantages in terms of cost and productivity. The applicant believes that such high precision is mainly due to the molding condition of the central zone of the tub, with high pressure and temperature which overcome problems of defects and shrinkage. Such degree of precision of the central part of the molded drum is identical or similar to the precision of machined metal surfaces. Moreover such precision is maintained even if the tub is stored in environment where the metal machined surface could be affected by corrosion or the like.

To the extent not already described, the different features and structures of the various embodiments can be used in combination with each other as desired. That one feature may not be illustrated in all of the embodiments is not meant to be construed that it cannot be, but is done for brevity of description. Thus, the various features of the different embodiments can be mixed and matched as desired to form new embodiments, whether or not the new embodiments are expressly described. All combinations or permutations of features described herein are covered by this disclosure.

While the invention has been specifically described in connection with certain specific embodiments thereof, it is to be understood that this is by way of illustration and not of limitation. Reasonable variation and modification are possible within the scope of the forgoing disclosure and drawings without departing from the spirit of the invention which is defined in the appended claims. 

What is claimed is:
 1. A washing machine with a direct drive motor system comprises: a plastic tub, a drum rotatably mounted inside the tub, and a shaft connected to the drum for transmitting a driving torque of a direct drive motor, the motor comprising a rotor assembly fastened to the shaft and a stator assembly fastened to the tub, wherein that the plastic tub has a central protruding portion upon which a central opening of the stator assembly is centered before it is fastened to the tub, and wherein the central opening of the stator assembly presents a plurality of centering surfaces configured to cooperate with an outside cylindrical surface of the central protruding portion of the tub.
 2. A washing machine according to claim 1, wherein the central protruding portion of the tub has a tapered part for pre-centering the stator assembly and a cylindrical portion for centering the stator assembly.
 3. A washing machine according to claim 2, wherein at least the central protruding portion of the tub is formed by an outer plastic shell co-injected onto a metal hub where bearings for the shaft are mounted.
 4. A washing machine according to claim 3, wherein the bearings are ball bearings mounted in the metal hub.
 5. A washing machine according to claim 1, wherein at least the central protruding portion of the tub is formed by an outer plastic shell co-injected onto a metal hub where bearings for the shaft are mounted.
 6. A washing machine according to claim 1, wherein the stator assembly is fixed to the tub by means of screws parallel to the shaft.
 7. A washing machine according to claim 6, wherein the screws are fastened to the metal hub.
 8. A washing machine according to claim 1, wherein the central protruding portion has at least one radial extension configured to cooperate with a corresponding seat of the stator assembly for assuring a determined angular position of the stator assembly with reference to the tub.
 9. A method for producing a washing machine with a direct drive motor system comprising a plastic tub, a drum rotatably mounted inside the tub and a shaft connected to the drum for transmitting a driving force of a direct drive motor, the motor comprising a rotor assembly fastened to the shaft and a stator assembly fastened to the tub, the method comprising: co-injecting the plastic tub on a metal reinforcing element which acts also as central hub so that a central protruding plastic portion is created as a plastic layer on one end of the hub, and axially sliding the stator assembly on the plastic layer in order to center the stator assembly, a plurality of centering surfaces provided on the central opening of the stator assembly cooperating with an outside cylindrical surface of the central protruding portion of the tub during the axially sliding. 